The present invention relates to a powder charging apparatus for charging powder paint that is necessitated when electrostatic powder painting is effected, and an electrostatic powder painting apparatus for applying powder paint charged by the powder charging apparatus to an article to be painted.
Heretofore, in such type of powder charging apparatus and in an electrostatic powder painting apparatus provided with th e powder charging apparatus, firstly, a ring-shaped electrode is provided on an inner peripheral surface of a tubular passage for conveying powder paint or the like as carried by gas, a tip end of a corona discharge electrode is disposed on the axis of the above-mentioned ring-shaped electrode to make an ionic current flow continuously from the corona discharge electrode towards the inner peripheral surface of the ring-shaped electrode, and powder flowing through the tubular passage simultaneously therewith is charged upon traversing the ionic current.
In addition, secondly, a ring-shaped slit is provided at an upstream end portion of the inner peripheral surface of the ring-shaped electrode, the opening of the slit is directed to the downstream side so that the inner peripheral surface of the ring-shaped electrode may be rubbed by clean gas ejected therefrom at a highvelocity, and thereby the inner peripheral surface can be always kept clean.
Furthermore, thirdly, the configuration of the ringshaped shaped electrode in each of the above-described apparatuses is chosen to form a cup-shaped cylindrical electrode diverging towards the downstream, and also at the downstream end of the inner peripheral surface of the cylindrical electrode is disposed a ring-shaped slit with its opening directed towards the upstream side.
As described above, in the first and second apparatuses in the prior art, when the powder flowing through the tubular passage traverses the radial ionic current, the powder is charged, but since this powder flowing through the tubular passage is apt to flow as deviated to one side of the inner peripheral surface of the tubular passage generally due to influences of the gravity and bending of pipings connected to the upstream, and moreover since the above-mentioned radial ionic current would have its ionic current density reduced as the position approaches from the axis to the inner peripheral surface and the powder paint would traverse the portion where the ionic current density is low, one cannot expect a high charging efficiency.
In addition, since clean air is ejected at a high velocity from the ring-shaped slit towards the downstream side, the velocity of the powder flowing through the tubular passage is increased, hense a stay time in the charging region becomes short, and the charging efficiency would be lowered. Also, when this charging apparatus is used in a spray gun of an electrostatic powder painting apparatus, the velocity of the spraying material ejected from the tip end of the gun becomes large, and so, there is a fear that a painting efficiency for an article to be painted may be lowered.
The third one of the techniques in the prior art is such that it is avoided for powder paint to pass through a region where an ionic current density is low, but the powder paint is made to traverse a region where the density is high, and thereby a charging efficiency is improved. Furthermore, the powder flowing through the tubular passage is decelerated by clean air at a high velocity that is ejected from a ring-shaped slit to thereby also improve the charging efficiency, and it is contemplated that when this charging apparatus is used in a spray gun of an electrostatic powder painting apparatus, the velocity of powder ejected from the tip end of the gun is not increased and thereby the painting efficiency of the powder paint to an article to be painted is enhanced. In this case, the clean air ejected at a high speed from the opening of the ring-shaped slit would flow along a cup-shaped inner surface of a cylindrical electrode from a large diameter portion towards a small diameter portion, because the opening is directed towards the upstream side. Thereafter, it collides and joints with powder material flowing through a tubular passage communicated with the small diameter portion towards the downstream, and subsequently flows as traversing an ionic current which flows radially from the corona discharge electrode towards the inner surface of the cylindrical electrode. At this time, since the clean air flowing along the inner surface of the cup-shaped electrode has its flow direction varied gradually from the upstream direction to the direction directed to the axis of the tubular passage as it moves from the large diameter portion to the small diameter portion, the flow of this clean air would guide the powder flowing through the tubular passage towards the downstream so as to make the powder approach from the inner periphery of the tubular passage to its axis. At this moment, since the above-mentioned clean air collides with the powder being conveyed and quickly stirs the powder, the powder can be well dispersed, and furthermore, due to the effect of the velocity component possessed by the clean air that is opposite in direction to the velocity of the conveying air, the powder is decelerated, so that the period when the powder stays in the region where the corona discharge is generated becomes long.
As a result, the powder flowing through the tubular passage is narrowed towards the axis of the tubular passage, hence the powder would traverse the central portion of the radial ionic current in a narrowed state, and it is charged while passing through the region of the radial ionic current where the current density is highest and the electric field strength is strongest.
The common point of the above-described techniques in the past is characterized in that according to anyone of the techniques, a normal operation is such that plasma is generated by only the corona electrode, a mono-polar ionic current drawn from the plasma flows continuously towards a ring-shaped electrode or a cylindrical electrode, and under this condition, powder to be charged is passed through the space intervening the respective electrodes.
Problems which are common to these techniques in the past, are that in the case of processing powder having a high specific resistance and a strong adhesiveness, as an operating time elapses the powder adheres to and accumulates on the surface of the ring-shaped electrode or the cylindrical electrode, due to back corona discharge generated here a large ionic current of the opposite polarity would flow towards the corona discharge electrode, thereby the charge accumulated by the corona discharge current would be neutralized, and charging of the powder becomes unstable and eventually impossible.